SRIM Special – Stopping in Compounds
The Stopping and Range in Compounds
Soon after the discovery of energetic particle emission from radioactive materials, there was interest how
these corpuscles were slowed down in transversing matter. In 1900 Marie Curie stated: "Les rayons alpha
sont des projectiles materiels susceptibles de perdre de leur vitesse en traversant la matière." [1] Scientists
realized that since these particles could penetrate thin films, experiments measuring their energy loss and
scatter might finally unravel the secrets of the atom. Bragg and Kleeman started to conduct such
experiments with a radium source in 1903, but were unable to find many types of thin films, since there
were no "goldbeaters" in Australia. So they studied the energy loss of alpha particles in hydrocarbon
gases such as methyl bromide and methyl iodide to find how alpha stopping depended on the atomic
weight of the target. They removed the stopping contribution of hydrogen and carbon atoms in the
hydrocarbon target gases by a subtraction of the stopping power in equivalent pure carbon and hydrogen
targets. From their analysis in 1905, comes Bragg's Rule that the stopping of a compound may be This
rule is reasonably accurate, and the measured stopping of ions in compounds usually deviates less than
20% from that predicted by Bragg's rule. The accuracy of Bragg's rule is limited because the energy loss
to the electrons in any material depends on the detailed orbital and excitation structure of the matter, and
any differences between elemental materials and the same atoms in compounds will cause Bragg's rule to
become inaccurate. Further, any bonding changes may also alter the charge state of the ion, thus changing
the strength of its interaction with the target medium.
Experimental studies of Bragg's rule started in the 1960s, and wide discrepancies were found from simple
Figure 1 : Atom Stopping in Simple Compounds
SRIM Special – Stopping in Compounds.doc 2/17/2016
page 1 of 13
United States Naval Academy
Annapolis, MD, USA
SRIM Special – Stopping in Compounds
additivity of stopping powers. See Figure 1. for an example of H and C non-additivity in simple
hydrocarbons as estimated by the linear combination of the stopping powers of individual elements.[2]
.[3]. In this figure, the stopping in various hydrocarbons is taken for pairs of compounds, and the relative
contribution of H and C is extracted (solving two equations with two unknowns). It is found that the
relative contribution of H and C differs by almost 2x over the range of compounds. Similar work studied
more complex hydrocarbons but instead of adding H and C bonds, they added extra hydrocarbon
molecules. In this study, they found that by adding molecules to hydrocarbon strings, stopping linearity
returned.[4]. See Figure 2. Adding new molecules just scaled the stopping by the extra number of atoms.
These results showed that atomic bonding had large effects on stopping powers of simple molecules while
extra agglomeration of
molecules had a small
Figure 2 : Molecule Stopping Large Compounds
stopping effect.
Since
these
early
experiments, theorists have
shown
that
extensive
calculations can predict the
stopping of light ions
(usually
protons)
in
hydrocarbon compounds.
Much of this work has been
based on a seminal paper by
Peter
Sigmund
that
developed
methods
to
account for detailed internal
motion within a medium.[5]
This theory allows for
arbitrary
electronic
configurations in the target.
Sabin and collaborators
used this approach to
calculate stopping powers
for protons in hydrocarbons
with
good
success.[6]
Sabin's calculation follows
what is sometimes called
the Köln Core and Bond
(CAB) approach which is
discussed in detail below.
We are not going to review
the complex theoretical
work on stopping in
compounds. Our interest is
to try to find a simple way
SRIM Special – Stopping in Compounds.doc 2/17/2016
page 2 of 13
United States Naval Academy
Annapolis, MD, USA
SRIM Special – Stopping in Compounds
to estimate the changes that compounds introduce to the use of Bragg's rule to generate stopping powers
of compounds. For the general user of SRIM, with only a minimal knowledge of the physics of stopping
theory, we need a simple method to estimate bonding corrections to stopping.
The Core and Bond (CAB) approach suggested that stopping powers in compounds can be predicted using
the superposition of stopping by atomic "cores" and then adding the stopping due to the bonding
electrons.[7] The core stopping would simply follow Bragg's rule for the atoms of the compound, where
we linearly add the stopping from each of the atoms in the compounds. The chemical bonds of the
compound would then contain the necessary stopping correction. They would be evaluated depending on
the simple chemical nature of the compound. For example, for hydrocarbons, carbon in C-C, C=C and
C=t=C structures would have different bonding contributions (C=C indicates a double-bond structure and
C=t=C is a triple bond). The contribution to stopping by a carbon atom in a C=C bond is almost twice that
of a carbon atom single-bond state. And a carbon atom in a triple-bond state contributes even greater
stopping powers. See Figure 3. By merely specifying the bonding of the atoms in the compound, for
example, SRIM can then generate a stopping correction required for the compound with this bonding
arrangement.
Figure 3 : Core & Bonds
SRIM uses this approach to generate
corrections between Bragg's rule and
compounds containing the common
elements in compounds: H, C, N, O, F, S
and Cl. These light atoms have the
largest bonding effect on stopping
powers. Heavier atoms are assumed not
to contribute anomalously to stopping
because of their bonds (discussed later).
When you use SRIM, you have the
option to use the Compound Dictionary
which contains the chemical bonding
information for about 150 common
compounds. The compounds with
corrections are shown with a Star
symbol next to the name. When these
compounds are selected, SRIM shows
the chemical bonding diagram and
calculates the best stopping correction.
The correction is a variation from unity
(1.0 = no correction). Notice that carbon
atoms have almost a 4x change in
stopping power from single bonds to
triple bonds. This large change indicates
the importance of making some sort of
correction for the stopping of ions in
compounds.
The CAB approach that SRIM uses has
SRIM Special – Stopping in Compounds.doc 2/17/2016
page 3 of 13
United States Naval Academy
Annapolis, MD, USA
SRIM Special – Stopping in Compounds
been tested on more than 100 compounds, from 162 experiments (discussed later). SRIM correctly
predicts the stopping of H and He ions in compounds with an accuracy of better than 2% at the peak of
the stopping curve, ~125 keV/u.
You can introduce other compounds to SRIM by adding to the Compound Dictionary. You need to edit
the SRIM file ..\Data\COMPOUND.dat. Instructions are at the top of the file, and included are more than
150 examples to guide you in creating your own compounds. SRIM will use your chemical bonding
information to calculate the stopping correction. SRIM can only calculate bonding of the seven elements:
H, C, N, O, F, S and Cl. Any other elements will be assumed to have the same stopping as in their
elemental form.
Details of how SRIM calculates the bonding correction to stopping can be found in the paper: J. F. Ziegler
and J. M. Manoyan, Nucl, Inst. Methods, B35, 215-228 (1988).
[1] Mme. Pierre Curie, C. R. Acad. Sci., 130, 76 (1900).
[2] W. H. Bragg and R. Kleeman, Philos. Mag., 10, 318 (1905)
[3] A. S. Lodhi and D. Powers, Phys. Rev., A10, 2131 (1974).
[4] D. Powers, Acc. Chem. Res., 13, 433 (1980).
[5] P. Sigmund, Phys. Rev., A26, 2497 (1982).
[6] J. R. Sabin and J. Oddershede, Nucl. Inst. Methods, B27, 280 (1987).
[7] G. Both, R. Krotz, K. Lohman and W. Neuwirth, Phys. Rev., A28, 3212 (1983).
Core and Bond Corrections
There have been more than 400 papers which discuss methods of calculating the stopping of ions in
compounds (Compound Citations listings). Many recent papers tend to be rather rigorous and apply
molecular orbital calculations to single systems. Our interest is to find an accurate solution which can be
used for any ion / compound situation with limited input from the user. The most basic information is still
necessary: the chemical nature of the compound. From this rudimentary information, SRIM then
estimates the corrections needed to produce stopping which is accurate to a few percent. (SRIM's input
requirements are discussed below.)
The method which is used is called the Core and Bond (CAB) approach, which was first proposed by a
group at the University of Cologne (Köln, Germany).[1] They proposed approaching the problem by
reducing each target atom to two parts: the core electrons which are unperturbed by bonding, and the
bonding electrons. An example would be for carbon atoms, with a core of the nucleus and the inner shell
electrons, and then separate outer shell contributions depending on how the carbon was bound into the
compound.
The problem is to determine the Core and Bond values for common elements. The Core stopping
contributions would simply follow Bragg's rule for the atoms of the compound, which suggests a linear
addition of the stopping from each of the elements in the compound. The chemical bonds of the
compound would then contain a correction to this basic stopping. The bonds would be evaluated
depending on the simple chemical nature of the compound. For example, for hydrocarbons, carbon in CC, C=C and C C structures would have different bonding contributions (C=C indicates a double-bond
structure and C C is a triple bond). The contribution to stopping by a carbon atom in a C=C bond is
almost twice that of a carbon atom single-bond state. And a carbon atom in a triple-bond state contributes
even greater stopping powers. By merely specifying the bonding of the atoms in the compound, for
example, SRIM can then generate a stopping correction required for the compound with this bonding
arrangement.
SRIM Special – Stopping in Compounds.doc 2/17/2016
page 4 of 13
United States Naval Academy
Annapolis, MD, USA
SRIM Special – Stopping in Compounds
The Core and Bond values may be determined by analyzing the stopping of ions over a great number of
targets, and solving for the contribution from the Cores and the Bonds. We use the stopping of light ions,
H, He and Li, in 162 different stopping experiments. The compounds contained 30 different elements. We
will assume that for the targets with 12 heavy elements, the stopping is dominated by the non-bonding
electrons (discussion below). But 114 of these compounds contained only light elements, and their
stopping is significantly altered by their bonding. The light elements we consider are: H, C, N, O, F, S and
Cl. For each of these elements there is a core contribution, and a separate bonding contribution. The bonds
which appear in the 114 compounds with only light elements that we studied were:
Compound Bonds which were Evaluated
(Table 1)
Hydrogen Carbon Other
Bonds
Bonds
Bonds
H-H
C-C
N N
H-C
C=C
N-O
H-N
C C
O=O
H-O
C-N
S-H
H-S
C-O
S-C
C=O
C-F
C-Cl
where "-" denotes a single bond, "=" denotes a double bond and " " denotes a triple bond. These 16 bonds
are considered unknown parameters. The scaling of ion stopping from H to He to Li are also considered
unknowns. Finally the stopping effect of the phase-state of the target is another unknown parameter. This
gives a total of (16 bonds) + (3 ions) + (2 phase states) = 20 parameters that will be extracted from the
data of 114 experiments. For H in targets, we assume there is no core (only one electron is available) and
all its stopping is in the bonding electron. For the other elements, each possible bond is considered
separately.
It is clear that we have an "over-defined" problem, in which there are many more equations than the
number of unknowns, and this improves the accuracy of the solutions. The solving of the multiple
equations was done in a standard iterative method for over-defined problems.
The limitations of this approach should be mentioned.
A. The most important limitation might be that of the target band-gap. Experiments on insulating
targets dominate the experimental results that we use. For compounds which are conducting, there
might be an error with the suggested stopping correction. It might be too low. Theoretically, band
gap materials are expected to have lower stopping powers than equivalent conductors because the
small energy transfers to target electrons are not available in insulators. It is not clear what the
magnitude of this effect is, but about 50 papers have discussed the stopping of ions in metals and
their oxides, e.g. targets of Fe, Fe2O3 and Fe3O4. (Examples are shown below: Bragg's Rule and
SRIM Special – Stopping in Compounds.doc 2/17/2016
page 5 of 13
United States Naval Academy
Annapolis, MD, USA
SRIM Special – Stopping in Compounds
Heavy Elements) These experiments evaluated similar materials with and without band-gaps. No
significant differences were found that could be attributed to the band-gap. Measurements have
also been made of the stopping of H and He ions into ice (solid water) with various dopings of salt
(NaCl). No change of energy loss was observed for up to 6 orders of magnitude change in
resistivity of the ice.
B. The scaling of ion stopping from H to He to Li is assumed to be independent of target material,
This assumption has been evaluated with 27 targets which have been measured for two of the three
ions (at the same ion velocity) and 6 of these targets have been measured for all three ions (see
listings in Table 2). In all cases, the stopping scaled identically within 4%. That is, for H (125
keV) and He (500 keV) and Li (875 keV) the scaling of stopping powers was 1 : 2.7 : 4.7 for the
27 targets (average error was <4%). (For those unfamiliar with stopping theory, the primary
parameter for the scaling of stopping powers is the ion velocity, which reduces to scaling in units
of kev/amu. This is reviewed in the section on Stopping History.)
C. The light elements of He and Ne are missing from the above list of target bonding atoms. No
comparative experiments have been done on the stopping into He in solid / gas phases. However
studies of stopping into targets of Ne and Ar have been conducted in both gas and solid form.
These papers show no significant difference between the stopping in gas and solid phases. It
appears that the van der Waals forces which hold noble gases together in frozen form, are too
weak to effect the energy loss of ions. Of particular note is the extensive work done in the paper:
F. Besenbacher, J. Bottiger, O. Graversen, J. Hanse and H. Sorensen, "Stopping Power of Solid
Argon for Helium Ions", Nucl. Inst. Methods, 188, 657-667 (1981).
D. The light target atoms of Li, Be and B are missing from the list of bonding atoms. This is a serious
defect. The number of papers that have looked at compounds which contain significant amounts of
these three elements is too limited to allow their evaluation. Target atoms of these three elements
are considered by SRIM to have no bonding correction, which is clearly not true. But without
experimental data, there is no reliable way to evaluate the contribution of their bonds in
compounds.
Reference [1] G. Both, R. Krotz, K. Lohman and W. Neuwirth, Phys. Rev., A28, 3212 (1983).
Table 2 shows the compounds used for the extraction of Core and Bond parameters.
Compounds used to extract Bonding Contributions (Table 2)
Compound
Acetaldehyde
Acetic Acid
Acetone
Acetylene
Alcohol, methyl-
Formula
Ions
Compound
Formula
Ions
C2H4O
He
Ethylene
C2H4
H, He, Li
CH3COOH
Li
Ethylene oxide
C2H4O
He
C3H6O
He, Li
Ethylene sulfide
C2H4S
He
C2H2
H, He
Formic Acid
HCOOH
Li
CH3OH
He, Li
Glycerol
C3H8O3
Li
SRIM Special – Stopping in Compounds.doc 2/17/2016
page 6 of 13
United States Naval Academy
Annapolis, MD, USA
SRIM Special – Stopping in Compounds
Alcohol, ethyl-
C2H5OH
He, Li
Hydrogen Sulfide
SH2
He
Alcohol, propyl-
C3H7OH
He, Li
Methane
CH4
H, He
Alcohol, undecanol-
C11H23OH
Methane, chloro-trifluoro-
CClF3
He
Allene
C3H4
He
Methane, dichloro-difluoro
CCl2F2
He
Aluminum oxide
Al2O3
H, He, Li
Methane, dichloro-fluoro-
CHCl2F
He
Ammonia
NH3
H
2-Methyl, 1,3-butadiene
C5H8
Li
Anthracene
C14H10
H
Methyl sulfide
(CH3)2S
He
Benzene
C6H6
H, He, Li
Nickel oxide
Ni2O3
H, He
Bicyclo[221]hepta2,5diene
C7H8
Li
Nitrous oxide
N2O
H, He
Butane
C4H10
He
Octanoic acid
C7H15COOH
Li
1,30Butadiene
C4H6
He
n-Pentane
C5H12
Li
2-Butanone
C4H8O
He
N-Pentadecane
C15H32
Li
Butyraldehyde
C4H8O
He
1,5-Pentanediol
C5H12O2
Li
Carbon tetrachloride
CCl4
He
3-Pentanone
C5H10O
He, Li
Carbon tetrafluoride
CF4
He
1-Pentene
C5H10
Li
C4H9Cl
Li
Phenylacetylene
C8H6
He
1-Chloroheexadecane
C16H33Cl
Li
Polyethylene
(CH2)n
H
1-Chlorohexane
C6H13Cl
Li
Polypropylene
(C3H6)n
H
2-Chloro-2-methylpropane
C4H9Cl
Li
Polystyrene
(C8H8)n
H, He
1-Chloropropane
C3H7Cl
Li
Propane
C3H8
H, He
1,3,5-Cycle-heptatriene
C7H8
Li
1,3-Propanediol
C3H8O2
Li
1,3-Cyclo-hexadiene
C6H8
He
2-Propanol
C3H7NH2
Li
Cyclohexane
C6H12
He, Li
Propylamine
C3H7NH2
Li
C6H10O
He
Propylene
C3H6
H, He
C6H10
He, Li
Propylene oxide
C3H6O
He
1-Chlorobutane
Cyclohexanone
Cyclohexene
SRIM Special – Stopping in Compounds.doc 2/17/2016
page 7 of 13
United States Naval Academy
Annapolis, MD, USA
SRIM Special – Stopping in Compounds
Cyclooctane
C8H16
He, Li
Propylene sulfide
C3H6S
He
Cyclopentane
C5H10
He, Li
Silicon dioxide
SiO2
H, He, Li
Cyclopentene
C5H8
He, Li
Thiophene
C4H4S
He
Cyclopropane
C3H6
H, He
Toluene
C7H8
He, Li
n-Decane
C10H22
Li
Trimethylene sulfide
C3H6S
He
1,2-Difluorethane
C2H4F2
He, Li
Water (solid)
H2O
H, He, Li
p-Dioxane
C4H8O2
He
Water (gas)
H2O
H, He, Li
C2H6
He
Hydrogen (gas)
H2
H, He
(CH2OH)2
Li
Nitrogen (gas)
N2
H, He
Ethane hexafluoride-
C2F6
He
Oxygen (gas)
O2
H, He
Ethane hexafluoride
C2F6
He
Graphite
C6
H, He
Ether, dimethyl-
C2H6O
He
Ether, vinyl-methyl-
C3H10O
He
Ether, diethyl-
C4H10O
He, Li
Ethane
1,2-Ethanediol
The simultaneous fitting of ion stopping in all of these compounds yielded the following Bonding
Corrections, see Table 3. The corrections are for H, He and Li ions at 125 keV/amu (about the peak of
their stopping power curve) and all strengths are normalized to the C-C bond. The evaluations were done
at several ion velocities, both higher and lower than 125 keV/amu, and the relative magnitude of the
bonds changed. The spread of the relative effects was significantly reduced for higher ion energies. For
lower energies, the spread of relative strengths slightly increased, but not very much.
Relative Strength of Bonding in Compounds (Table 3)
Hydrogen Relative Carbon Relative
Bonds
Strength
H-H
2.44
C-C
1.00
N#N
5.17
H-C
1.83
C=C
2.49
N-O
4.01
H-N
2.09
C#C
3.81
O=O
5.40
H-O
2.22
C-N
1.29
S-H
1.23
H-S
1.23
C-O
15.7
S-C
0.41
C=O
3.53
SRIM Special – Stopping in Compounds.doc 2/17/2016
Bonds Strength
Other Relative
page 8 of 13
Bonds Strength
United States Naval Academy
Annapolis, MD, USA
SRIM Special – Stopping in Compounds
C-F
2.79
C-Cl
0.94
The relative strength of the target atom "cores" were also extracted, see Table 4. The contribution of the
hydrogen core was set to zero because its only electron is used in bonding. All core strengths are shown
relative to carbon. The core contribution to stopping was found to be relatively independent of the ion
velocity, in contrast to the contribution of bonding, above.
Relative Strength of Atomic Cores (Table 4)
Target Atom
Relative Core
Strength
Hydrogen
0.000
Carbon
1.000
Nitrogen
0.93
Oxygen
0.89
Fluorine
0.88
Sulphur
5.33
Chlorine
4.69
Bragg's Rule and Heavy Target Elements
We have concentrated on the analysis of the stopping of ions in compounds made up of light elements.
For compounds with heavier atoms, many experiments have shown that deviations from Bragg's rule
disappear. In Table 5 are shown representative examples of ion stopping in various compounds containing
heavy elements. None show measurable deviations from Bragg's rule. The citations for these
measurements are tabulated in the Compound Citation page. Many articles with similar results were
reviewed in the 1980s.[1,2]
Bragg's Rule Accuracy in Heavy Compounds (Table 5)
Compound
Deviation from
Bragg's rule
Compound
Deviation from
Bragg's rule
Compound
Deviation from
Bragg's rule
Al2O3
< 1%
HfSi2
< 2%
Si3N4
< 2%
Au-Ag alloys
< 1%
NbC
< 2%
Ta2O5
< 1%
Au-Cu alloys
< 2%
NbN
< 2%
TiO2
<1%
BaCl2
< 2%
Nb2O5
< 1%
W2N3
< 2%
SRIM Special – Stopping in Compounds.doc 2/17/2016
page 9 of 13
United States Naval Academy
Annapolis, MD, USA
SRIM Special – Stopping in Compounds
BaF2
< 2%
RhSi
< 2%
WO3
< 2%
Fe2O3
< 1%
SiC
< 2%
ZnO
< 1%
Fe3O4
< 1%
For compounds which contain elements with atomic numbers greater than 12, it is possible to combine the
CAB approach with Bragg's rule. The CAB approach can be used for the small atomic number cores and
bonds, and these can be combined with the normal stopping contribution of the other components of the
compound.
[1] D. I. Thwaites, Nucl. Inst. Methods, B12, 84 (1985).
[2] D. I. Thwaites, Nucl. Inst. Methods, B27, 293 (1987).
Examples of Stopping Correction for Compounds
Stopping Correction for a target of Ethylene
When you use SRIM, you are given the option of using the Compound Dictionary. This menu lists about
200 compounds, and provides stopping corrections for a great number of them. Below is a typical
example for Ethylene, C2H4, which has a total 12% stopping correction. Below on the left is the SRIM
Compound Dictionary window for Ethylene. The Bonding Correction is 8.33%. Below this is the density
for Ethylene in gas phase, and the chemical formula and the bonding information in schematic form.
Below this is the composition of Ethylene in both atomic percent and mass percent. The term "Core
Stopping" is the same as the Table 4 above, but in different units. Finally, at the bottom the bonding
information is listed. There are four (H-C) single bonds, and one (C=C) double bond. The effect of these
cores and bonds is to make a 8.3% correction. SRIM also makes an automatic correction for the phase
change for carbon in the target if the "Gas Phase" box is checked in SRIM (we are assuming gaseous
Ethylene in this example). The gas-phase correction is about 4% for carbon. This makes the total
correction to be 12%. Shown in figure He into Ethylene (gas) is the stopping of He ions into Ethylene
showing the Bragg's rule stopping estimate for (2 Carbon) + (4 Hydrogen) atoms (black curve). These
values are clearly too small. There are two corrections that are made. The stopping of He in Carbon
assumes a solid-phase target. The stopping in gas-phases usually increases the stopping. Shown in the
curve is the stopping due to carbon solid (solid green line) and carbon in gas phase (dashed green line).
Then we must consider the bonding effects. The ethylene molecule contains 4 H-C single bonds, and a
C=C double bond. From the discussion above, the C=C bond adds significantly to the stopping power
near the peak of the stopping. SRIM calculates that this increase is 8.3%. So the two corrections make the
total adjustment to the stopping to be about 12%, and it brings the calculation into reasonably accurate
agreement with the data.
Extensive plots of stopping in Compounds, as well as citations to more than 200 scientific papers which
have discussed the stopping of ions in compounds, may be found at the website:
http://www.srim.org/SRIM/Compounds.htm
SRIM Special – Stopping in Compounds.doc 2/17/2016
page 10 of 13
United States Naval Academy
Annapolis, MD, USA
SRIM Special – Stopping in Compounds
Ethylene
Stopping Correction for Target Chemistry
Solid Target = +10.79%
Gas Target = + 3.04%
=======================================
Density = 0.00125 g/cm3
Chemical H
H
Formula
\
/
C == C
C H
/
\
2 4
H
H
------ TARGET COMPOSITION -------Atom Atom Number Atom
Core
Name Numb Atoms Mass % Stopping
---- ---- ------- ------ -------H 1 4.00 14.37
0.00
C 6 2.00 85.63
6.21
-TARGET BONDS (per molecule)Bond Type Number Stopping
--------- ------ -------(H-C)
4
7.438
(C=C)
1 10.023
SRIM Special – Stopping in Compounds.doc 2/17/2016
page 11 of 13
United States Naval Academy
Annapolis, MD, USA
SRIM Special – Stopping in Compounds
Stopping Correction for a target of Polystyrene
Another example of a large correction is that necessary for a target of Polystyrene, C 8H8. Shown below is
the Compound Dictionary in SRIM for Polystyrene (which is also identified by the ICRU #226). This
compound will have two corrections, one to convert stopping in H (gas) to stopping in H (solid). This will
tend to decrease the stopping power. The second correction will be for the bonding of the compound,
which will increase the stopping. The result will be a net increase of about 6%. The chemical form of
Polystyrene is rather complex since it contains 8 (H-C) bonds, 6 (C-C) bonds and 3 (C=C) bonds. Note
that the density of Polystyrene is quite variable, and you must be careful that your target density is
correctly entered.
Shown in figures "H into Styrene" and "He into Styrene" are the stopping of H and He ions into
Polystyrene. They show the Bragg's rule stopping estimate for (8 Carbon) + (8 Hydrogen) atoms (black
curve). These values are too small. There are two corrections that must be made. The stopping of ions in
Hydrogen assumes a gas-phase target. The stopping in gas-phases has higher stopping than for solid
phase. Shown in the curve is the stopping due to hydrogen in gas phase (dashed green line) and hydrogen
solid (solid green line). Then we must consider the bonding effects. The Polystyrene molecule contains 8
H-C single bonds, 6 C-C single bonds, and 3 C=C double bonds (see the molecular structure in SRIM's
Compound Dictionary). From the discussion above, the C=C bond adds significantly to the stopping
power near the peak of the stopping. SRIM calculates that the bonding correction is +6.6%. In this case
the two corrections work opposite to each other. The phase-change correction for hydrogen reduces the
stopping, while the bond correction increases the stopping. The total adjustment to the stopping is about
6%, and it brings the calculation into reasonably accurate agreement with the data for both H and He
ions.
SRIM Special – Stopping in Compounds.doc 2/17/2016
page 12 of 13
United States Naval Academy
Annapolis, MD, USA
SRIM Special – Stopping in Compounds
SRIM Compound Stopping Correction
Polystyrene (ICRU-226)
Stopping Correction for Target Chemistry
Solid Target = +8.59%
Gas Target = +0.99%
=======================================
Density = 1.06 g/cm3
Chemical H – C
- C - H
Formula
/ \
|
H - C
C - C - H
|C H |
\\
//
|
| 8 8|n
C - C
H-C-H
|
|
|
H
H
|
Density ranges from 0.98 to 1.075 g/cm3.
-------- TARGET COMPOSITION --------Atom Atom Number Atom
Core
Name Numb Atoms Mass % Stopping
---- ---- -------- ------- -------H 1
8.00
7.74
0.00
C 6
8.00 92.26
6.21
-TARGET BONDS (per molecule)Bond Type Number Stopping
--------- ------ -------(H-C)
8
7.438
(C-C)
6
3.964
(C=C)
3
10.023
SRIM Special – Stopping in Compounds.doc 2/17/2016
page 13 of 13
United States Naval Academy
Annapolis, MD, USA